Various methods currently exist for producing porous or dense substrate bodies. Sintering methods are generally based on parameters that accelerate pressing of the material that is initially more or less porous. Heat-treating prepressed molded parts made of fine materials, in particular powders, is considered to be sintering, as a result of which solid metal or ceramic parts having precisely defined dimensions and properties can be produced.
One particular production engineering method, in which powders and solids, especially ceramics and metals, are simultaneously hot-pressed and sintered, is hot isostatic pressing (HIP). In this method, the component to be produced is placed in a heatable pressure vessel. The component is pressed under inert gas or technical air at temperatures of up to 2000° C. and pressures of from 100 to 350 MPa. Since the pressure acts on the workpiece from all sides, the very dense component produced displays isotropic properties.
The disadvantages of this method are the high manufacturing costs and large restrictions with regard to the dimensional tolerance achievable. The main field of use of the HIP method is for pressing encapsulated powders and for repressing metal and ceramic workpieces that have already been sintered.
JP 2006 045038 A, for example, thus discloses a method for producing a metal or ceramic component, in which the component is produced from powder inside a drum mold, under pressure and at a high temperature, by means of two molding tools. The component has preferably the shape of a biconvex lens with a flat edge.
It has also proven advantageous for a pressing process for an electric field to also be applied to the material to be sintered during the exertion of pressure, so that the material heats up by means of what is known as the Joule effect (resistive heating as a result of a current flowing over the component or press insert).
A method that takes advantage of this knowledge is field assisted sintering technology/spark plasma sintering (FAST/SPS). These relate to a sintering method that is comparable to hot pressing and can be carried out under inert gas, in a vacuum or in air.
The advantages of the FAST/SPS method are the very steep heat and cooling ramps of up to 1000 K/min in combination with short holding times, which overall lead to considerably shorter processing times and very effective pressing of more than 90% of the theoretical density. Furthermore, when using powders having particle sizes in the nanometer range, this nanostructuring can often be maintained since an increase in particle size is generally prevented due to the short processing times. Diffusion induced by an electric field also appears to be possible [1].
A typical device for carrying out a FAST/SPS method is shown in FIG. 1. The material to be sintered (metal or ceramic) is usually poured into an electrically conductive die in powder form, which is generally made of graphite and is currently most commonly in the shape of a hollow cylinder. The material is prepressed by means of two pressurized punches, which are generally also made of graphite and are precisely adapted to the hollow-cylindrical geometry of the crucible. This is followed by the actual pressing step (sintering step) in an SPS machine. Here, a hydraulic press is used to exert a defined pressure on the punch. This step can take place under inert gas and in a vacuum.
The components that can be produced by means of such a method generally have a prismatic or cylindrical shape, the two bases of the components generally being parallel to one another, the two bases usually having a congruent geometry and the lateral face(s) being perpendicular to the base.
Unlike with hot pressing, in which the required sintering temperature is provided by an external heat supply, in FAST/SPS methods the material to be sintered is heated up by a pulsed current or by a direct current having current strengths of typically a few kiloamperes and a voltage of a few volts.
If the powder is a conductive powder, the current pulse is directly conducted through the die and through the powdery material, both heating up due to the ohmic resistance. In order to directly heat the powder, said powder needs to be sufficiently electrically conductive.
If the powder is a non-conductive powder, the current pulse is directly conducted through the die so that said die is heated first of all, and the powder in the die is then also heated by means of thermal conduction.
The FAST/SPS method is generally used for pressing metal or ceramic materials, which initially exist as a powdery starting material. Furthermore, the method is also used for rapid pressing in cases when problems occur during the sintering process, for example in metal powders that tend to form stable oxide layers.
The FAST/SPS method could therefore be used to successfully produce high-density aluminum alloys [2].
At the same time, the FAST/SPS method was used to successfully produce porous metal carriers from NiCoCrAlY powders, which are suitable for use as membranes [3].
Proton-conducting membranes are normally used for separating gases. Gas-separation membranes are generally used to separate desirable components from a gas flow, such as gas molecules, oxygen ions or protons. Substrate-assisted fuel cells and electrolytic cells are used for generating energy in an energy-efficient manner or for producing Hz. The metal or ceramic substrates used for these applications typically have a porosity of between 15 and 40 vol. %. In this case, the porosity should be as high as possible in order to keep the gas transport resistance through the membrane as low as possible.
On a laboratory scale, such membranes having diameters of approximately 15 mm are typically installed in a reactor and are sealed at the edges by means of gold rings. Under test conditions of from 800 to 1000° C., the gold rings advantageously soften and thereby allow for a perfect seal between the membrane and the reactor housing. Gas is thereby only transported via the membrane.
On an industrial scale, in which the membranes have considerably larger dimensions, a very effective seal is likewise required between the membrane and the gas-tight housing in order to prevent uncontrolled gas transport and gas exchange between the feed stream and the permeate side. For this purpose, the metal-carried or ceramic-carried membrane can be sealed by means of either solid phase sinter processes or liquid phase processes [4].
Diffusion bonding is the preferred method from the field of solid phase sintering and can be used for sealing both metal and ceramic components. However, the long processing times, high pressure and high temperatures can lead to further pressing of the porous membrane, which disadvantageously leads to an increased gas flow resistance.
In the liquid phase processes, a differentiation is made between soldering methods and welding methods. Soldering methods using glasses or metal-based solders can preferably be used for metal and ceramic carriers, whereas welding methods can only be used for metal carriers. In general, both soldering and welding methods involve problems when used with porous materials, since the liquid phases produced can be easily transported through the pore structure due to capillary forces. This can, in turn, disadvantageously lead to the pores being partially closed and to a geometrically undefined weld seam. Examples of this include the formation of gaps in an incomplete weld seam on the one hand, or, on the other hand, the formation of a concave depression in the weld seam when the molten phase penetrates the pore structure adjacent to the weld seam. Such irregularities in the weld seam are difficult to coat with a membrane material, since the risk of the formation of cracks in these regions is normally increased.
Various welding techniques, such as gas tungsten arc welding, inert-gas welding, electron-beam welding or laser welding are already used to produce membrane reactors [4].
In arc welding, pores in the weld seam can be enclosed. In this case, extensive heat affected zones (HAZ) are often also formed. In electron-beam welding and laser welding, smaller amounts of heat are introduced than in arc welding, and therefore only smaller heat affected zones are produced, too. In these heat affected zones, a disadvantageous growth in particle size and/or phase transformations often occurs. This generally leads to warpage stress and distortion at the boundaries, which adversely affect the stability and dimensional stability of the component. In addition, heat affected zones have a disadvantageous effect on the homogeneous distribution of the alloy elements in the metal substrate and the adjacent, gas-tight components of the membrane reactor housing. A change in the composition of the alloy both in and near to a weld seam can, in turn, significantly reduce the corrosion resistance of the metal substrate and the feed.